Overall, let me say I'm not sure why the behavior changed on you.
Review your application logs for notes about user sessions being removed. If the Mendix runtime is removing them, it will log a note about it that will say something like:
Removing session for user 'bob'
If that's happening, it's because the user's session timed out. There are custom runtime settings (detailed here) that are relevant in this case:
Especially if EnableKeepAlive is disabled, there's nothing keeping a user's session alive while they're on a page unless they're executing microflows or retrieving data. That could cause unexpected timeouts. Similarly, if for some reason these keepalive calls are blocked by the user's browser for some reason, that could cause an issue, as they're responsible to refreshing the session on the server.
The other thing that comes to mind is security. Do you have 'Project -> Security -> Check security' enabled? If not, I think then it's possible that user is redirected to a login screen when they try to access a resource they aren't allowed to. I can't confirm that behavior without some testing, however.
Hope that helps!